Parametric Gear Toy

A parametric gear toy based on the [parametric spur gear](https://www.thingiverse.com/thing:3336641). The four gear teeth counts are adjustable parameters. The default settings yields a gear ratio of approximately 3 from the outside to inside gear. Three M5 threaded thumbscrews keep the gears in place on their shafts.

See the [parametric bearing gear toy](https://www.thingiverse.com/thing:3336671) for a bearing based version.

This design exposes the following top-level parameters:

* `TO